In an effort to enhance the level of comfort for the occupant of a seat, it is possible for the seat to be designed in such a way as to provide what might be called an air-conditioning effect, insofar as the seat can provide for a flow of air through a part thereof so as to improve what might be termed the climate of the seat. That consideration may be particularly important for example in relation to a vehicle seat in which seat occupant comfort can be a matter of paramount importance. In one form of a cushion portion for a seat with an air-conditioning zone, as disclosed for example in DE 36 26 984 C1, which is particularly intended for a vehicle seat, the cushion portion has a cover comprising at least two cover portions which are connected together along common edges, with at least one cover portion which is impervious to air having a plastic foam backing thereon. To produce the cushion portion the interconnected cover portions are fitted into a mold and secured thereto by a clamping frame structure. The mold is then closed by means of a mold cover and the cover portions forming the cover for the seat portion are caused to bear snugly against the walls of the mold cavity. A reaction mixture is then introduced into the mold, the reaction mixture foaming up as the reaction takes place and filling the mold. The cushion portion can be removed from the mold after the reaction has occurred, to produce a foam filling for the cushion portion. The cushion portion and the process for producing same also involves the use of a second cover portion for the cushion portion, which is permeable to air. An insert portion is disposed on the or each permeable cover portion, and thereupon a foil which is impermeable to air is fixed to the permeable cover portion in the vicinity of the or each insert portion, and is arranged to cover the peripheral surface of the insert portion. The foil is then secured to the cover member of the mold and the mold is then closed by the cover member. After the reaction of the reaction mixture has taken place, that is to say after termination of the reaction mixture foaming process and after the cushion portion has been removed from the mold, a separate cushion portion is disposed in the space in the previously molded cushion, which is formed by the insert member. Accordingly it will be seen that that procedure involves a first molding operation to produce a first cushion portion which is of a generally frame-like configuration with a space therein, and a second operation in which a separate cushion portion is fitted thereinto. That process therefore involves a number of working steps and that has an effect on the production costs of the cushion portion. It is also necessary to provide a base element to receive the two cushion portions.